1. Field of the Invention
The present invention relates generally to symbol timing offset estimation and, more particularly, to a method and apparatus for estimating a symbol timing offset in an Orthogonal Frequency Division Multiplexing (OFDM) based communication system.
2. Description of the Related Art
Much research has been conducted in order to provide users with high speed data services satisfying various Quality of Service (QoS) requirements. Particularly, research has been focused on the improvement of the high speed data communication services while guaranteeing mobility and QoS in a Broadband Wireless Access (BWA) communication system, such as a Wireless Local Area Network (WLAN) or a Wireless Metropolitan Area Network (WMAN). In order to achieve a high speed data rate over wired and wireless communication channels, OFDM is one of the most promising technologies. As one of the MultiCarrier Modulation (MCM) schemes, OFDM converts a serial input symbol stream into parallel streams and modulates the parallel streams orthogonally into multiple subcarriers.
In the wireless channel environment, unlike the wired channel environment, the transmission signal is likely to be erroneous due to various factors such as multipath interference, shadowing, attenuation, time-varying noise, Intersymbol Interference (ISI) caused by delay spread, and frequency selective fading, resulting in data loss at the receiver. Typically, in a wireless communication system, multipath fading channels are generated between the transmitter and the receiver in the wireless communication system due to various obstacles therebetween. In such a multipath channel environment, the transmission signal is received via multiple paths that are created due to the reflection of the signal from obstacles. The start point of a symbol should be determined in consideration of the multipath propagation so as to minimize the ISI caused by the multipath channels. The process for finding the start point is referred to as a symbol timing offset estimation or symbol synchronization process.
Typically, the symbol timing offset is estimated using a Reference Signal (RS) carried by pilot tones, while receiving downlink signals to find the start point of symbol. However, the RS-based symbol synchronization is likely to cause ambiguity of symbol timing offsets due to the repetition components, as shown in FIG. 1, especially in the multipath fading channel environment in which the maximum delay spread is relatively large. The maximum delay spread is the time taken until the last reflection signal is received after the receiver starts receiving the signal.
FIG. 1 is a graph illustrating channel components of a conventional OFDM system in a multipath fading channel environment having a large delay spread. As shown in FIG. 1, in the multipath-fading channel environment characterized by the large delay spread, repetition components 110 appear according to the structural characteristic of the RF pilot signals, as well as the real channel components. When an observation window 101 is fixedly set with its center positioned where Inverse Fast Fourier Transform (IFFT) output occurs, the repetition components 110 appear prior to channel components 120 of the real signal in the range of the observation window 101 such that the repetition components 110 are likely to be recognized as the first arrival path. Accordingly, the signal received through the repetition components is misunderstood as the signal of the real channel path, thereby causing ambiguity of timing offsets in the symbol timing offset estimation process.
In the multipath channel environment, the multipath fading channel environment is aggravated due to a high Doppler frequency shift as the movement speed of the mobile terminal increases. In such a case, ghost components of the signal, in addition to the repetition components, may appear due to the interpolation error on the time axis of the RF as shown in FIG. 2.
FIG. 2 is a graph illustrating channel components of a conventional OFDM system in a multipath fading channel environment having high Doppler shift frequencies.
As shown in FIG. 2, in the multipath fading channel environment characterized by high Doppler frequencies, the repetition components caused by the structural characteristic of the RF pilot signals and ghost components 210 appear as well as real channel components 220. Particularly, when an observation window 201 is fixedly set with its center positioned where the IFFT output occurs, the ghost components 210 appear prior to the channel components 220 of the real signal in the range of the observation window 201. However, the conventional OFDM-based communication system does not provide a specific process for processing the ghost components. When the mobile terminal moves fast, the symbol timing offset estimation performance of the mobile terminal is significantly deteriorated due to the increase of the ghost components. Accordingly, there have been efforts to reduce the deterioration of the symbol timing offset estimation performance that is caused by the repetition components and/or the ghost components.